Broadband spectrum illumination sources are commonly implemented in spectroscopic based optical metrology tools. Such broadband based metrology tools may be utilized to measure various parameters associated with a given sample, such as a semiconductor wafer or lot of semiconductor wafers. Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices.
One type of broadband based metrology includes spectroscopic ellipsometry (SE). Spectroscopic ellipsometry (SE) is commonly implemented in order to measure thicknesses of film stacks on semiconductor wafers and critical dimensions (CD) of semiconductor structures in a variety of semiconductor manufacturing processes. One type of SE system includes a rotating polarizer SE (RPSE), which typically includes a broadband light source (e.g., xenon lamp), a rotating polarizer, a fixed analyzer (may be rotated at discrete angles), and a spectroscopic detector. RPSE is an extremely powerful tool, initially used for film thickness measurements, and is now widely applied to measure both film thicknesses and critical dimensions of samples. Another type of SE system includes the rotating compensator spectroscopic ellipsometer (RCSE), which typically includes a broadband light source, a fixed polarizer, a rotating waveplate (i.e., “compensator” in ellipsometry), a fixed analyzer, and a spectroscopic detector.
As critical dimensions (e.g., the gate width or gate oxide thickness) continue to decrease, measurement of these ever shrinking dimensions (e.g., a few nanometers or sub-nanometer) requires improved measurement performance. One of the fundamental performance requirements includes measurement precision, or the so called “3-sigma” of the measured CD parameters. An additional challenge includes tool-to-tool matching. Both requirements are related to the sensitivity of the implemented measurement system. As the system parameters decrease, the measurable signal change, corresponding to the fractional change in given parameters, are often not distinguishable from system noise for commonly implemented standard RPSE and RCSE systems. As a result, prior art implementations are insufficient both in terms of precision and tool-to-tool matching capability.
One method to improve measurement sensitivity includes the measurement of an increased number of spectral properties associated with geometric and material structures of a given sample. In an RPSE system, a motor is used to drive a polarizer with angular frequency ω. The signal measured by the detector of the system consists of three harmonic terms, known as the DC term, and terms related to cos 2 ωt and sin 2 ωt, respectively. In practice, the DC term is used to normalize the cos 2 ωt and sin 2 ωt harmonics, resulting in the commonly termed [α, β] spectra. As a result, one can measure two sets of combinations of Mueller elements, normalized by the first Mueller element (M00). Similarly, in an RCSE system, one may obtain three normalized harmonics, and is then able to measure three combinations of Mueller elements normalized by the first Mueller element (M00).
A common approach to measuring additional Mueller elements includes Mueller SE, also referred to as “complete SE,” which includes the implementation of two rotating polarizing elements. One such complete SE system includes a dual-rotating compensator SE (DRCSE) system, which includes a light source, a fixed polarizer, a first rotating waveplate in the illumination path, a second rotating waveplate in the collection path, a fixed analyzer, and a detector. This complete SE system configuration is capable of collecting up to 25 harmonic coefficients, corresponding to certain linear combinations of the rotation frequencies of these two rotating compensators. As a result, all 15 normalized Mueller elements can be resolved and measured.
Enhanced sensitivity of Mueller SE is advantageous for measuring small CD parameter changes. Nevertheless, this enhanced sensitivity will also cause the given measurement system to be more susceptible to system errors, due to the sensitivity of the additional Mueller elements to perturbations in the system. As a result, improved systems and methods for calibration of critical ellipsometry parameters are desirable.